(1) Field of the Invention
This invention relates to radiographic apparatus for use in the medical field, and in industrial fields for performing a non-destructive testing, RI (Radio Isotope) inspection or optical inspection.
(2) Description of the Related Art
A conventional apparatus of this type includes an X-ray tube and a flat panel X-ray detector (hereinafter called “FPD” as appropriate) opposed to each other, and performs a primary scanning by rotating the X-ray tube and X-ray detector together about a sectional axis (also called a primary scanning axis) passing through a site of interest of an object under examination, and a secondary scanning by rotating the X-ray tube and X-ray detector together about an axis (also called a secondary scanning axis, and hereinafter referred to as “body axis”) extending substantially perpendicular to the sectional axis. The apparatus acquires a three-dimensional sectional image based on a group of projection data obtained from the FPD at varied points of time during the primary scanning and secondary scanning (as disclosed in Japanese Unexamined Patent Publication No. 2004-141656, for example).
However, the conventional apparatus of such a construction has the following drawback.
When performing a reconstruction process, a three-dimensional lattice is virtually set to the site of interest of the object under examination. This three-dimensional lattice has one side thereof extending parallel to the body axis of the object. Information is given to each lattice point of the three-dimensional lattice based on projection data at a point where an X ray passing through that lattice point falls on the detecting plane of the FPD (hereinafter called simply “projection point of the lattice point”). This process is called back projection. The FPD has detecting elements arranged in matrix form on the detecting plane, and a detection signal is acquired from each detecting element. Projection data at the projection point of each lattice point is derived from detection signals acquired from a plurality of (e.g. four) detecting elements around the projection point.
Consider lattice points arranged in one row extending parallel to the body axis. The projection points of these lattice points are distributed on one straight line. Since the FPD rotates about the sectional axis, this straight line rotates 360 degrees on the detecting plane of the FPD in one primary scan. Therefore, the projection points hardly become parallel to a direction of a row or column of the detecting elements, but are distributed among three or more rows or three or more columns of the detecting elements.
The projection points of lattice points arranged in directions not parallel to the body axis also are distributed among three or more rows or three or more columns of the detecting elements.
In time of calculation, on the other hand, access is made beforehand to a relatively low-speed main memory that stores detection signals, and required detection signals are taken into a relatively high-speed cache memory by designating a start address and range of the detection signals. However, signals from a large number of rows will exceed the capacity of the cache memory. In this case, it is necessary to access the main memory again. It can be advantageous from the viewpoint of effectively using the capacity of the cache memory to divide detection signals into blocks and to take in only required detection signals. However, the main memory must be accessed the number of times corresponding to the number of blocks. Since access to the main memory requires a relatively long time, the greater accessing frequency results in the longer time required for a reconstruction process.